"They are able to live and thrive in one of the world's harshest environments. But there's concern that their habitat may literally be melting."

The Endangered Species Act requires federal agencies to ensure that all activities the government approves will not harm listed species or their habitats.

Environmental groups quickly connected the announcement with scientific evidence that climate change is melting the iconic bear's Arctic habitat, causing the animals to go hungry and give birth less often.

"This is a watershed decision," said Kassie Siegel, an attorney for the Center for Biological Diversity in Joshua Tree, California. "Even the Bush Administration can no longer deny the science of global warming."

The center was one of three organizations that filed suit against the administration to protect the bear from the impacts of global warming in the Arctic.

The proposed listing was published today in the Federal Register. Public comments will be accepted for 90 days, and a final listing decision is expected within a year.

Regulating Emissions?

A formal listing of polar bears as a threatened species would raise the possibility that the U.S. government will force curbs on greenhouse gas emissions, Siegel says.

Polar bears live only in the Arctic, the northernmost region of Earth. The bears swim between ice floes to hunt their primary prey, the ringed seal. They also travel, mate, and sometimes give birth on the ice.